Adding jansson-2.4

Jansson is a small library for parsing and writing JSON documents. The specfile is [from Fedora 19][1], and works for EL6 without modification. I built for EL6, x86_64 and i386 without any problems.

 [1]: http://pkgs.fedoraproject.org/cgit/jansson.git/tree/jansson.spec?id=4d5715b0a3547444eb3d4563078755ba3b95bc78